Thecal designs in isorophinid edrioasteroids

摘要

A study of functional morphology in the edrioasteroid suborder Isorophina reveals three major thecal designs: domal, pyrgate, and clavate. Pyrgate forms are characterized by specialized morphologic features that facilitate extension of the theca well above the substrate and contraction of the theca to a low, streamlined, domal profile. Domal forms have a low, domal thecal profile capable of little shape change. Clavate forms are characterized by modifications to the pyrgate theca to facilitate higher‐level suspension feeding. Pyrgate species are generalized edrioasteroids that inhabited a variety of nearshore open‐marine environments. Domal species are small specialized edrioasteroids that inhabited shallow‐marine to intertidal environments. Clavate species inhabited offshore, open‐marine environments. The size reduction of the peripheral rim and the modifications to the pedunculate zone are adaptations allowing clavate edrioasteroids to compete for small attachment sites while maintaining the ability to extend the theca for higher‐level suspension feeding and respiration. A phylogenetic analysis of well‐known isorophinid edrioasteroids to determine the relationships between the three designs indicates that domal species are a monophyletic group within the Isorophinidae, clavate species are a monophyletic group within the Agelacrinitidae, and isorophinid pyrgate species are a paraphyletic group ancestral to both domal and clavate forms. □Edrioasteroidea, Echinodermata, functional morpholo